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Wastewater treatment plants (WWTPs) are a point source of nutrients, emit greenhouse gases (GHGs), and produce large volumes of excess sludge. The use of aquatic organisms may be an alternative to the technical post-treatment of WWTP effluent, as they play an important role in nutrient dynamics and carbon balance in natural ecosystems. The aim of this study was therefore to assess the performance of an experimental wastewater-treatment cascade of bioturbating macroinvertebrates and floating plants in terms of sludge degradation, nutrient removal and lowering GHG emission. To this end, a full-factorial experiment was designed, using a recirculating cascade with a WWTP sludge compartment with or without bioturbating Chironomus riparius larvae, and an effluent container with or without the floating plant Azolla filiculoides, resulting in four treatments. To calculate the nitrogen (N), phosphorus (P) and carbon (C) mass balance of this system, the N, P and C concentrations in the effluent, biomass production, and sludge degradation, as well as the N, P and C content of all compartments in the cascade were measured during the 26-day experiment. The presence of Chironomus led to an increased sludge degradation of 44% compared to 25% in the control, a 1.4 times decreased transport of P from the sludge and a 2.4 times increased transport of N out of the sludge, either into Chironomus biomass or into the water column. Furthermore, Chironomus activity decreased methane emissions by 92%. The presence of Azolla resulted in a 15% lower P concentration in the effluent than in the control treatment, and a CO2 uptake of 1.13 kg ha-1 day-1. These additive effects of Chironomus and Azolla resulted in an almost two times higher sludge degradation, and an almost two times lower P concentration in the effluent. This is the first study that shows that a bio-based cascade can strongly reduce GHG and P emissions simultaneously during the combined polishing of wastewater sludge and effluent

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Structurally complex habitats, such as mangrove forests, allow for rich assemblages of species that benefit from the provided space, volume and substrate. Changes in habitat complexity can affect species abundance, diversity and resilience. In this study, we explored the effects of habitat complexity on food web networks in four developmental stages of mangrove forests with differing structural complexities: climax > degrading > colonizing > bare, by analyzing food web structure, stable isotopes and habitat complexity. We found that food webs became gradually more biodiverse (species richness: +119 %), complex (link density: +39 %), and robust (connectance: −35 %) in climax versus bare stages with increasing complexity of the mangrove forest (i.e., number of trees, leaf cover, and pneumatophore densities). This study shows that habitat complexity drives food web network structure in dynamic mangrove forests. We recommend restoration practitioners to use this food web network approach to quantify habitat restoration successes complementary to traditional biodiversity metrics.

During the 1980s–1990s, 4600 hectares of the seagrass Zostera marina were permanently lost from Lake Grevelingen (the Netherlands), and restoration is planned. In 1971, the lake was fully marine (salinity 30), and seagrass covered 1300 hectares. After closure in that year, the lake gradually became brackish (salinity of 23 by 1978), and the meadows expanded to 4600 hectares. However, with the creation of a sluice connection to the sea in 1978, the lake returned to marine conditions and a fatal decline initiated. We revisit traditionally suggested causes of the disappearance of the seagrass, finding them unsatisfactory. We hypothesize that during the lower salinity conditions from 1971 to 1978, selection of low-salinity genotypes occurred, and these genotypes were not adapted to returning marine conditions. This hypothesis is no longer testable through genetic analysis in Lake Grevelingen but is supported by previously unpublished experiments that found a lack of seed germination at even moderately high salinity for the now extinct population. Such processes could be relevant for, and tested in, environmentally modified water systems worldwide, particularly when isolated. Based on our assessment, the abiotic environment of Lake Grevelingen seems suitable for Z. marina restoration using a donor from a high salinity environment.

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De ecologische kwaliteit van het Markermeer is sinds de aanleg van de Houtribdijk sterk achteruitgegaan. Die Houtribdijk weer weghalen is geen optie, al was het maar omwille van de ‘ecosysteemdiensten’, zoals de waterhuishouding. In 2016 is een alternatieve aanpak gekozen: uit de zachte sedimenten van het kunstmatige Markermeer is een archipel van vijf eilanden aangelegd: Marker Wadden. Het Markermeer kreeg daarmee weer geleidelijke land-waterovergangen, verschillende waterdiepten en beschutting tegen golven en sedimentophoping. Dit artikel beschrijft de eerste resultaten. Het voedselweb wordt ‘van onderaf’ hersteld door de primaire productie via algen te verbeteren, waar ook hogere planten en dieren van profiteren. Dit project illustreert hoe herstel van ecosystemen in meren kan worden bereikt met behoud van bestaande ecosysteemdiensten.

Greenhouse gas (GHG) emissions from small inland waters are disproportionately large. Climate warming is expected to favor dominance of algae and free-floating plants at the expense of submerged plants. Through different routes these functional plant types may have far-reaching impacts on freshwater GHG emissions in future warmer waters, which are yet unknown. We conducted a 1,000 L mesocosm experiment testing the effects of plant type and warming on GHG emissions from temperate inland waters dominated by either algae, free-floating or submerged plants in controls and warmed (+4 °C) treatments for one year each. Our results show that the effect of experimental warming on GHG fluxes differs between dominance of different functional plant types, mainly by modulating methane ebullition, an often-dominant GHG emission pathway. Specifically, we demonstrate that the response to experimental warming was strongest for free-floating and lowest for submerged plant-dominated systems. Importantly, our results suggest that anticipated shifts in plant type from submerged plants to a dominance of algae or free-floating plants with warming may increase total GHG emissions from shallow waters. This, together with a warming-induced emission response, represents a so far overlooked positive climate feedback. Management strategies aimed at favouring submerged plant dominance may thus substantially mitigate GHG emissions.

Globally, peatlands have been affected by drainage and peat extraction, with adverse effects on their functioning and services. To restore peat-forming vegetation, drained bogs are being rewetted on a large scale. Although this practice results in higher groundwater levels, unfortunately it often creates deep lakes in parts where peat was extracted to greater depths than the surroundings. Revegetation of these deeper waters by peat mosses appears to be challenging due to strong abiotic feedbacks that keep these systems in an undesired bare state. In this study, we theoretically explore if a floating peat mat and an open human-made bog lake can be considered two alternative stable states using a simple model, and experimentally test in the field whether stable states are present, and whether a state shift can be accomplished using floating biodegradable structures that mimic buoyant peat. We transplanted two peat moss species into these structures (pioneer sp. Sphagnum cuspidatum and later-successional sp. S. palustre) with and without additional organic substrate. Our model suggests that these open human-made bog lakes and floating peat mats can indeed be regarded as alternative stable states. Natural recovery by spontaneous peat moss growth, i.e., a state shift from open water to floating mats, is only possible when the water table is sufficiently shallow to avoid light limitation (<0.29 m at our site). Our experiment revealed that alternative stable states are present and that the floating structures facilitated the growth of pioneer S. cuspidatum and vascular plants. Organic substrate addition particularly facilitated vascular plant growth, which correlated to higher moss height. The structures remained too wet for the late-successional species S. palustre. We conclude that open water and floating peat mats in human-made bog lakes can be considered two alternative stable states, and that temporary floating establishment structures can induce a state shift f

Aquatic ecosystems provide vital services, and macrophytes play a critical role in their functioning. Conceptual models indicate that in shallow lakes, plants with different growth strategies are expected to inhabit contrasting habitats. For shallow peat lakes, characterized by incohesive sediments, roles of growth forms, life-history strategies and environmental factors in determining the occurrence of aquatic vegetation remain unknown. In a field survey, we sampled 64 points in a peat lake complex and related macrophyte occurrence to growth forms (floating-leaved rooted and submerged), life-history strategies for overwintering (turions, seeds, rhizomes) and environmental factors (water depth, fetch, and porewater nutrients). Our survey showed that macrophyte occurrence relates to water depth, wind-fetch, and nutrients, and depends on growth form and life-history strategies. Specifically, rooted floating-leaved macrophytes occur at lower wind-fetch/shallower waters. Submerged macrophytes occur from low to greater wind-fetch/water depth, depending on life-history strategies; macrophytes with rhizomes occur at greater wind-fetch/depth relative to species that overwinter with seeds or turions. We conclude that growth form and life-history strategies for overwintering predict macrophytes occurrence regarding environmental factors in peat lakes. Therefore, we propose an adapted model for macrophyte occurrence for such lakes. Altogether, these results may aid in species-selection to revegetate peat lakes depending on its environment.

Ecosystems are increasingly managed to provide multiple benefits to humans, which often degrades their ecological integrity. This strongly applies to aquatic ecosystems, in which engineering can enhance flood protection, drinking water supply, fisheries and recreation. Although these activities typically increase ecosystem functionality to humans, they often impair key aspects of biodiversity and natural functioning.Classical restoration of such degrading freshwater ecosystems can lead to societal opposition, if returning to a former ecosystem state affects previously acquired ecosystem services. Innovative nature-based solutions are therefore needed that enhance natural values in ecosystems, without affecting existing services.We present a large-scale project aiming to increase the ecological integrity of a human-modified freshwater lake while maintaining its services to humans. The freshwater lake Markermeer in the Netherlands was formed by closing off an estuary for flood protection. The ecological integrity of this lake diminished over time, likely because a declining primary productivity impaired biodiversity at higher trophic levels. This decline is associated with a lack of gradual land–water transitions, strong resuspension of fine sediments, low nutrient availability and lack of dynamics typically to be expected in a natural temperate freshwater lake. Restoring the lake to its former marine state would conflict with current ecosystem services.A nature-based solution was initiated in 2016, consisting of constructing a five-island archipelago from the lake's own soft-sediments called the ‘Marker Wadden’. The project aims to increase the lake's primary production by creating gradual land–water transitions, more heterogeneity in water depths and decreasing turbidity by creating shelter and deep sinks reducing fine-sediment resuspension by the wind – thus introducing currently missing elements that are typical for natural lakes. We present the underlying ecologi

Due to high waste sludge disposal at wastewater treatment plants and increasing phosphorus scarcity, there is a need to combine waste removal and nutrient recovery. One way to achieve this is by incorporating ecological processes into wastewater treatment. Positive species interactions, such as facilitation, are critical to increase productivity of biomass and nutrient recovery. In this study we showed the potential of using ecological principles including interspecific facilitation processes of aquatic plants (Azolla filiculoides) and worms (Oligochaeta, Tubificidae) in waste recovery and biomass production. This was investigated by developing a biocascade with monocultures of plants and aquatic worms that was fed on activated sludge. Tubificidae had an average relative growth rate of 0.02 g g−1 DW d−1 whereby sludge predation resulted in 45% sludge reduction. When Tubificidae were present in the biocascade, A. filiculoides biomass production significantly increased to a relative growth rate of 0.15 g g−1 DW d−1. The activity of Tubificidae mostly affected total suspended solids, chemical oxygen demand and ammonium concentration in the first compartment of the biocascade. Additionally, nitrification rates increased and the water acidified, leading to increased carbon dioxide concentrations and dissolved phosphorus-binding metals (zinc, iron, aluminium and manganese) that stimulated A. filiculoides growth. The high sludge reduction (45%) and phosphorus sequestration (133 mmol m−3 d−1) show a strong potential of the biocascade for combined sludge waste reduction and phosphorus recovery from wastewater.